During a weaving operation for forming a woven fabric web on a loom, the warp threads are subjected to different warp tension variations, particularly during the changing or alternation of the shed. Namely, when the shed is open for the purpose of the weft insertion and beat-up, the warp tension is at a maximum value, and during the change or alternation of the shed the warp tension is at a minimum value. This alternating tension applied to the warp leads to relatively high loading and demands placed on the warp threads, which give rise to warp thread breaks. Moreover, the reduced or minimum warp tension that is established during the change of shed can impair the proper guidance and manipulation of the warp threads. Namely, if the warp thread tension is too low and therefore all or particular ones of the warp threads are too slack, the respective warp threads can become entangled or hung-up on each other and thereby impair the proper opening of the next shed. Such tangling or sticking of the warp threads are also often the cause of warp thread breaks and can lead to so-called weft loops or weft fold-backs in the case of looms using a pneumatic weft insertion.
The above described problems have been addressed in looms using a closed-shed shed formation for carrying out a linen weave or plain weave operation. In such a case, the above mentioned problems have been at least partially resolved by supporting the backrest either fixedly or rotatably on swing arms, such that the backrest carries out controlled or uncontrolled periodic swinging movements for compensating the warp tension variations during the shed changes. Such methods and arrangements are disclosed in German Patent 32 16 903, German Patent 35 32 798, and European Patent Application 0,409,306, for example.
The above described problems regarding the varying tension load on the warp threads become even more pronounced and more difficult to resolve when weaving a fabric using more than one weave binding pattern or binding type. One typical case is when the fabric edges are to have a different weave binding pattern in comparison to the rest of the fabric. For example, if the fabric edges are to be woven with a 2/2 weave binding pattern in order to provide a strong and stable fabric edge, while the main body width of the fabric shall have a so-called linen weave or plain weave, i.e. a 1/1 weave binding pattern, then the respective groups of warp threads forming the fabric edges on the one hand, and forming the main body width of the fabric on the other hand, will be subjected to different degrees of tension variations between the open shed condition and the closed shed condition. If a uniform tension compensation is applied to all of the warp threads, then the tension of one of the thread groups will be overcompensated while the tension of the other thread group will be undercompensated. In any event, the above mentioned interfering problems relating to tangling of the warp threads, and a resulting disruption of the weaving process, will still arise in at least one of the groups of warp threads.
German Patent Laying-Open Document 26 59 530 discloses a loom in which the backrest is so arranged to carry out a certain translation movement for the purpose of compensating tension variations, which arise in the warp threads guided over the backrest as a result of the successive shed changes. Additionally, an auxiliary guide for a number of fabric edge warp threads is provided near at least one end of the backrest. This auxiliary guide is arranged and adapted to carry out a translational movement independently of the backrest, under the influence of an adjustable load acting thereon. Due to its independent translational movement, this auxiliary guide can easily follow and thereby compensate the tension variations that result from the successive shed changes. Thus, the actually arising tension variations in the respective fabric edge warp threads can be considerably reduced. However, also in view of the independent arrangement and motion of the auxiliary guide that is allocated to the warp threads of the fabric edge in combination with the tension compensating apparatus allocated to the backrest itself, the overall arrangement results in a considerable additional cost, effort, and complexity in the manufacturing, operation, and maintenance of the loom.